1. Field of Invention
The present invention relates to a process for the preparation of stable white grape juice. More particularly, the invention relates to a process which precludes the use of sulfites, thereby allowing enzymatic browning to proceed. The invention additionally relates to a process in which nitrogen sparging is used to control oxidation of the white grape juice produced according to the claimed process. The invention further relates to a process in which the white grape juice is stored aseptically.
2. Description of Related Art
White grape juice as currently manufactured comprises juice from white grapes of the V. labrusca species and V. viniferaspecies, such as, but not limited to, the hybrid Niagara, and the Thompson Seedless, French Collombard, and Muscat varieties. While commercial white grape juice can be produced from a single variety of grapes, blends are often employed to take advantage of the most desirable attributes of two or more varieties. For example, attributes such as sweetness, tartness, color, flavor, astringency, and viscosity vary significantly among different varieties of white grapes. By blending juices, one may achieve a finished white grape juice product with organoleptic characteristics superior to those found in white grape juice prepared from a single grape variety.
Conventional manufacturing processes employed in the commercial production of white grape juice require the addition and maintenance of efficacious levels of a sulfiting agent such as potassium metabisulfite to grapes and grape juice throughout harvest, pressing, and subsequent stages of manufacturing. The purpose of sulfite use is to inhibit degradation of color and flavor caused by classical enzymatic, non-enzymatic, and oxidative reactions. Such reactions are commonly associated with fruit and fruit juice browning. White grape juices are especially prone to loss of quality when not sulfited. In addition to sulfites, stabilizers or preservatives such as ascorbic acid and its salts, erythorbic acid and its salts, L-cysteine, and poly(vinyl-polypyrrolidone) have also been added in an attempt to preserve color and flavor.
Enzymatic browning is the result of the reaction of naturally occurring phenolase enzymes and juice substrates in the presence of oxygen. Non-enzymatic browning mechanisms include such well-known phenomena as caramelization of natural sugars, decomposition of ascorbic acid, and the Maillard Reaction.
In the typical processes of the prior art, a solution of potassium metabisulfite, hereafter referred to as sulfite, is added to the grapes during the mechanical harvesting of the grapes as each one-ton receptacle or bin is filled. Sufficient sulfite is added to achieve 200-300 parts per million (ppm) of SO.sub.2 in the mixture of grapes and free-run grape juice as delivered from the mechanical harvester to each bin.
The bins of sulfited grapes are covered and transported to a processing facility for pressing. Upon delivery to the processing facility, grapes are dumped and prepared for pressing by first removing stems and by crushing to facilitate juice removal.
The process of preparation and of pressing white grapes and filtration of grape juice prior to pasteurization is commonly referred to as "cold-pressing" and is typically carried out at ambient temperatures ranging from 50.degree.-90.degree. F. without employing pectinase enzymes and heat. A modification of the cold-pressing process employs pectinase enzyme or warming of grapes above 80.degree. F. but less than 100.degree. F. to facilitate juice expression.
De-stemmed grapes and juice are placed in holding kettles where additional sulfite is added to restore the SO.sub.2 level to 150 ppm to further protect against enzymatic browning. Cellulose fiber or rice hulls are also added to the sulfited slurry as pressing aids. This slurry is then passed over drag screens to remove free-run juice with the remaining pulps and cellulose fiber or rice hulls being sent separately to mechanical presses for juice expression.
Free-run juice from the screening process is merged with the juice stream from the presses, combined with additional cellulose fiber, and vacuum-filtered to remove a major fraction of insoluble substances.
Just before pasteurization, additional sulfite is added to ensure maintenance of SO.sub.2 levels of 150-200 ppm in the pasteurized juice after it reaches the storage tank.
To ensure microbiological stability in the storage tank, filtered, sulfited white grape juice is pasteurized, or heat-treated, at 185.degree.-192.degree. F., cooled to 30.degree.-32.degree. F., and pumped to temperature controlled (30.degree.-32.degree. F.) storage tanks where it is treated with pectinase and amylase enzymes for clarification. This is followed by a gradual settling of bitartrates (argols) and insoluble substances over a 1-2 month period.
After clarification and settling has been achieved, clear juice is drawn from storage tanks, blended with white grape juice from concentrate and sufficient sulfite is added to achieve 75-105 ppm SO.sub.2 in the finished, bottled white grape juice product. The juice blend is pasteurized, bottled or canned, cooled, and stored at ambient temperature for distribution and consumption.
Typical shelf-life for bottled or canned white grape juice which has been sulfited at 75-105 ppm SO.sub.2 ranges from 9-12 months. Without sulfites, severe loss of flavor and browning occur, rendering white grape juice organolepticaily unsuitable for commercial use after only 1-3 months at ambient temperature.
For many potential consumers, the presence of sulfites in white grape juice is unacceptable. Substitutes for sulfites have been the subject of extensive research in the scientific community. No suitable chemical alternative has been identified for fruit juices. Although several moderately successful chemical preservatives have been found useful in other foods, these have not been satisfactory substitutes for sulfites in juices and wines.
A search for alternatives to sulfites has led the present inventors to the development of a unique process for the preparation of commercial white grape juice. In one embodiment, the presently claimed process precludes the use of sulfites, thus allowing natural enzymatic browning to proceed. In yet another embodiment, dissolved oxygen in the product is controlled through nitrogen sparging. In a further embodiment, microbiological degradation is controlled by aseptic storage of the processed juice. The resulting finished bottled or canned white grape juice has a distinctive fruity character and is generally darker in color than sulfited white grape juice. The product prepared by this new process exhibits organoleptic stability up to 12 months at ambient temperature, resulting in a commercially acceptable product.
In yet another optional embodiment, very low levels of sulfites may be added just prior to or after heat treatment to obtain 30 ppm or less sulfites in the stored juice.